1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Related Art
The automatic transmission of the prior art is equipped with a torque converter acting as a fluid coupling for receiving the rotation generated by an engine, and a transmission for changing the speed of the rotation transmitted from the torque converter. The transmission is equipped with a planetary gear unit composed of a plurality of gear elements for effecting the shifting in accordance with a shift pattern which is preset to correspond to the vehicle speed, the throttle opening and so on.
The automatic transmission allows for selection of a (parking) P-range, a (reverse) R-range, a (neutral) N-range, a (drive) D-range, a (second) S-range, a (low) L-range and so on. If the N-range is switched to the D-range by operation of the shift lever, for example, the rotation of the engine at idling is transmitted through the torque converter to the transmission, to cause a "creep phenomenon" in which the vehicle will advance little by little without any depression of the accelerator pedal.
When a range such as the D-range, the S-range or the L-range (hereinafter a "forward range") for running the vehicle forward is selected, a determination of satisfaction of the following conditions is made: (1) that the accelerator pedal is released, (2) that the brake pedal is depressed and (3) that the vehicle speed is substantially "0". If these conditions are found to be satisfied, the forward clutch which is applied in a forward range (hereinafter "first clutch"), is released or brought into slipping engagement to establish a pseudo-neutral state (hereinafter "neutral control state") so that the aforementioned creep phenomenon may be prevented (as disclosed in U.S. Pat. No. 4,850,251).
The pressure of the oil fed to the hydraulic servo of the first clutch is controlled by a linear solenoid valve which generates a control pressure for actuation of a regulator valve which, in turn, generates a regulated oil pressure. A change-over valve selectively feeds either the regulated oil pressure or the D-range pressure to the hydraulic servo.
In the aforementioned control system of the prior art, the oil pressure fed to the hydraulic servo of the first clutch can be adjusted to smooth the engagement of the first clutch when the range is switched from the N-range to the D-range (or forward range). However, when the regulated oil pressure or the D-range pressure is selected and fed to the hydraulic servo by the change-over valve, shock in engagement of the first clutch may occur in switching from the N-range to the D-range, responsive to operation of the shift lever. Specifically, at the time of switching from the N-range to the D-range (hereinafter "N-D switching"), the manual valve is switched by the operation of the shift lever to generate the D-range pressure. Then, the D-range pressure is regulated by a regulator valve to generate a regulated oil pressure, and the change-over valve is switched to feed the regulated oil pressure to the hydraulic servo so that the first clutch can be smoothly applied.
If the switching of the change-over valve is delayed in the above described operation, the D-range pressure is temporarily fed through the change-over valve to the hydraulic servo, which is designed to have a stroke loss so that no influence is effected by receipt of the D-range pressure. However, if the aforementioned N-D switching is effected immediately after switching from the D-range to the N-range, the oil at the time of selecting the D-range remains in the hydraulic servo at that N-D switching time. If, therefore, the D-range pressure is temporarily fed to the hydraulic servo through the change-over valve, due to a delay in the switching of the change-over valve, the oil pressure in the hydraulic servo prematurely rises to cause a serious engaging shock.
FIG. 13 is a graph of oil pressure in the first clutch and torque (vertical axis) versus time (horizontal axis) in the control system for an automatic transmission of the prior art. As seen in FIG. 13, if the N-D switching is effected immediately after switching from the D-range to the N-range (hereinafter "D-N switching"), the oil pressure P in the hydraulic servo of the first clutch temporarily rises, so that the torque T abruptly rises to cause engaging shock.
The foregoing suggests the possibility of keeping the change-over valve switched to the regulated oil pressure selecting side, even when the N-range is selected by the shift lever. If such a design were adopted, even with D-range pressure generated in N-D switching, because the change-over valve would be switched to the regulated oil pressure selecting side, the hydraulic servo would receive the regulated oil pressure and engaging shock would be avoided. However, since a solenoid valve is used for switching the change-over valve, this solenoid valve would be turned ON and energized, not only while the pseudo-neutral state is established, but also when the N-range is selected. As a result, the power consumption would increase and, still worse, the solenoid would generate heat, shortening its useful life.